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I am studying blend modes and I am comfortable with pixels, channels and math equations. I understand that the screen blend mode is calculated as 1 - (1 - c1) * (1 - c2), where c1 and c2 are the colors (or more precisely, the value of each RGB channel).
The question is, why is it called "screen"?
It's called screen because of the analog process it mimics:
From Photoshop Blend Modes Explained :
As an analogy, imagine the selected layer and each of the underlying layers as being 35mm slides, and each slide being placed in a separate projector (one slide for each projector), then all of the projectors are turned on and pointed at the same projector screen…this is the effect of the Screen blend mode.
As the comments note, though, this is a bit too simplistic. The actual screen process effect/equations is more accurately like stacking negatives in an enlarger (or on a flatbed scanner these days).
